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A 55-year-old man presented with painless enlargement of the left testis. A preoperative ultrasound and subsequent orchiectomy showed 4.5 cm mass in the testis. It was well circumscribed, brown and without necrosis.

Archive Case and Diagnosis:
This case first appeared as Performance Improvement Program in Surgical Pathology (PIP) 2008, Case 35, and is Leydig cell tumor.

Criteria for Diagnosis and Comments:
This is a Leydig cell tumor which is the most common pure testicular sex cord-stromal tumor (1-3% of testicular tumors). They occur at any age but most commonly in adults at 30-60 years of age; 15-20% occur in prepubertal boys at age 5-10 years. The vast majority of Leydig cell tumors are unilateral and benign. Only 3% are bilateral.

Children usually present with features of isosexual precocity including deepening of the voice, development of body hair, penile enlargement, and advanced bone age. Adults commonly present with painless testicular enlargement and many times bilateral gynecomastia which may precede the presence of a palpable testicular mass. Leydig cell tumors produce a number of steroid hormones. This tumor must always be included in the differential diagnosis of any patient who presents with feminizing or virilizing symptoms. Since it may be small and non-palpable, an ultrasound of the testes might be required to detect the tumor.

The histologic features of Leydig cell tumor are very distinctive. The tumor cells grow in nests, small clusters or trabeculae; occasionally a microcystic pattern is noted focally or may predominate. The cells are generally large and polygonal with abundant eosinophilic cytoplasm and indistinct cell borders; nuclei are round with fine granular chromatin and a single prominent nucleolus. Some of the tumor cells may have distinct cell borders and smaller nuclei. Occasionally, the tumor cells are small with scant dense eosinophilic cytoplasm. Spindled shaped cells may be seen focally but rarely there is a predominant pattern of spindle cells forming a sarcomatoid pattern. Occasionally, lipid accumulation results in pale to clear cytoplasm. Lipofuschin pigment is commonly seen as a golden yellow to brown cytoplasmic pigment on hematoxylin and eosin stained sections. Rare cases of Leydig cell tumor have been reported with adipocyte differentiation, stromal ossification or calcification.

Reinke crystals are an important distinctive feature seen in up to 40% of Leydig cell tumors. They are pale, refractile crystals in the cytoplasm of the tumor cells; intranuclear Reinke crystals have also been reported. The may be round, needle-like or rhomboid. They are highlighted by a Masson trichrome or Giemsa stain. They are best seen on Giemsa air dried smears; fixed tissues and smears are not as sensitive since fixation dissolves the crystals.

Leydig cell tumors are immunoreactive with inhibin, calretinin, melan-A and vimentin. Inhibin is the best stain to differentiate Leydig cell tumor from germ cell tumors.

Leydig cell tumors in children are almost always benign. In adults, most Leydig cell tumors are also benign; mitoses are rare and nuclear atypia is absent or only focal. Benign tumors are usually less than 5 cm and have sharply circumscribed borders. However, 10% of Leydig cell tumors in adults are malignant; these tend to be larger than 5 cm and have an infiltrative margin. Malignant microscopic features include: cytologic and nuclear atypia, necrosis, lymphovascular invasion and increased mitotic rates (>5 mitoses per 10 hpf). An aneuploid DNA content has been shown to be more closely associated with malignant behavior. Additionally, there is increased expression of the proliferation marker MIB-1 in malignant Leydig cell tumors compared to their benign counterparts. Of the Leydig cell tumors which are clinically malignant, 20% present with metastases. Regional lymph nodes, liver and bone are favored sites of metastasis. Most patients with metastasis die within 5 years.

Leydig cell hyperplasia, especially when florid, may make one concerned about the possibility of Leydig cell tumor. Leydig cell hyperplasia is seen in atrophic testes, cryptorchid testes and Klinefelter syndrome. It is also seen in testicular parenchyma adjacent to germ cell tumors. Leydig cell hyperplasia does not form an expansile mass as in Leydig cell tumor, but rather is a multifocal interstitial process; the seminiferous tubules are spared and not displaced or destroyed which is an important diagnostic feature. Identifying atrophic tubules among the Leydig cells nodules will lead to the correct diagnosis.

Malakoplakia may form a distinct yellow to brown testicular mass; there may be an associated abscess. Microscopically the eosinophilic histiocytes seen under low power can be confused for Leydig cells. There is a mixture of other inflammatory cells in the interstitium and there may be a prominent intratubular involvement. An important diagnostic feature is the presence of cytoplasmic inclusions (Michaelis-Gutman bodies) which may be enhanced with Von Kossa staining for calcium.

Testicular tumor of the adrenogenital syndrome, or congenital adrenal hyperplasia, is a very important consideration in the differential diagnosis. The syndrome is the result of one of five possible enzyme deficiencies; 90% of cases are caused by 21-hydroxylase deficiency. It is the most common cause of ambiguous genitalia in infants. Constant ACTH secretion results in adrenal hyperplasia and hyperplasia of heterotopic adrenal cortical tissue. The tumor occurs in young adults (average age 22 years) and usually situated close to the testicular hilum. Bilaterality is frequent, >80%. It is composed of pleomorphic cells with eosinophilic cytoplasm which may easily be confused with the appearance of a Leydig cell tumor. Commonly, there is a hyalinized fibrous stroma. The tumor cells usually have abundant lipofuschin pigment. Well preserved seminiferous tubules may be present within the tumor while they are only rarely seen in Leydig cell tumor. Reinke crystals are not seen in testicular tumors of the adrenogenital syndrome but they are seen in up to 40% of Leydig cell tumors. Bilaterality is a very important clue to the diagnosis of congenital adrenal hyperplasia since Leydig cell tumors are usually unilateral. The tumor regresses after administration of corticosteroids.

Large cell calcifying Sertoli cell tumor is also in the differential diagnosis. The average age at presentation is 21 years. They may be multifocal and 20% are bilateral. Bilaterality is almost always seen in cases of Carney syndrome. This type of Sertoli cell tumor has large cells with abundant eosinophilic cytoplasm. The tumor cells grow in sheets, nests and cords; focal solid tubules are commonly present. An intratubular growth seen in 50% of cases is not a feature of Leydig cell tumor or other sex cord-stromal tumors. Calcification is usually conspicuous and an important defining characteristic. Sertoli cell tumors that occur in males with Peutz-Jeghers syndrome may have features of large cell calcifying Sertoli cell tumor except for the lack of calcification.